External css in iframe doesn't work

Description:
I use iframe on my page with absolute path on the same domain, iframe has src like /preview?parameter=1. In iframe I use css with absolute path like /desktop.bundles/folder/file.css witch has a class for iframe body to set it margin to 0. But in inspector in developer tools body styles doesn't have css from that class. In all other browsers everything works fine, I used to use iframe attributes marginwidth and marginheight set to 0 to work around this bug. Also js in my iframe works fine and not applyed css is the only problem. Also there are no problems on network tab and ccs file has 304 status. Opening iframe src in new tab works fine like expected and the bug not reproducable all the times, sometimes iframes works fine.

Steps to reproduce:

use iframe with same origin and absolute path on the page, src like /preview?parameter=1